The fabrication of a semiconductor device involves a plurality of discrete and complex processes. To perform these processes, a workpiece is typically disposed on a platen. The platen may be an electrostatic clamp or ESC, designed to retain the workpiece through the application of electrostatic forces produced by electrodes within the platen.
These electrostatic clamps are typically designed to be slightly smaller in diameter than the workpieces that they support. This insures that the electrostatic clamp is not exposed to the incoming ion beam. Contact with the ion beam could cause the generation of contaminants, or may do damage to the electrostatic clamp.
In addition to retaining the workpiece in place, the electrostatic clamp may also serve to heat the workpiece. Specifically, the electrostatic clamp is typically a larger mass of material, capable of supplying heat to the workpiece in other embodiments. In certain embodiments, the electrostatic clamp has conduits on its upper surface which supply a back side gas to the space between the upper surface of the ESC and the back surface of the workpiece. The electrostatic clamp may also have an outer seal ring near the outer edge, which serves to confine the back side gas in this volume and minimize back side gas leakage. The outer seal ring extends upward from the upper surface of the ESC and contacts the workpiece, forming a wall that contains the back side gas. This outer seal ring is effective because it contacts the workpiece. However, this outer seal ring is not disposed at the outer edge of the workpiece. Furthermore, it some embodiments, it is desirable to tilt the workpiece relative to the ion beam. This may be accomplished by tilting the electrostatic clamp, which may cause the ESC to become exposed to the ion beam. To minimize this exposure, the electrostatic clamp may be constructed to be smaller than the workpiece that is disposed thereon.
Because the electrostatic clamp is somewhat smaller than the workpiece, the outer edge of the workpiece may not be heated as effectively by the electrostatic clamp. Thus, in embodiments where the electrostatic clamp supplies heat to the workpiece, the outer edge of the workpiece may be cooler than the rest of the workpiece.
This difference in temperature may impact the yield of the workpiece. Therefore, it would be beneficial if there were an electrostatic clamp capable of achieving better temperature uniformity across a workpiece, in embodiments where the workpiece is heated by the ESC.